Mussel‐inspired biomaterials: From chemistry to clinic

Abstract After several billions of years, nature still makes decisions on its own to identify, develop, and direct the most effective material for phenomena/challenges faced. Likewise, and inspired by the nature, we learned how to take steps in developing new technologies and materials innovations. Wet and strong adhesion by Mytilidae mussels (among which Mytilus edulis —blue mussel and Mytilus californianus —California mussel are the most well‐known species) has been an inspiration in developing advanced adhesives for the moist condition. The wet adhesion phenomenon is significant in designing tissue adhesives and surgical sealants. However, a deep understanding of engaged chemical moieties, microenvironmental conditions of secreted proteins, and other contributing mechanisms for outstanding wet adhesion mussels are essential for the optimal design of wet glues. In this review, all aspects of wet adhesion of Mytilidae mussels, as well as different strategies needed for designing and fabricating wet adhesives are discussed from a chemistry point of view. Developed muscle‐inspired chemistry is a versatile technique when designing not only wet adhesive, but also, in several more applications, especially in the bioengineering area. The applications of muscle‐inspired biomaterials in various medical applications are summarized for future developments in the field.


| INTRODUCTION
From antiquity and ancient times, humankind has always been inspiring by the nature to make proficient use and design materias. Natural evolution has spent millions of years balancing and optimizing almost everything. Correspondingly, every aspect of natural phenomena has been inspired by scientists to mimic the nature from chameleons to leaves. Scientists have deeply been mimicking mechanisms underlying natural changes to find an optimum and lasting solution for addressing different issues engaged with the human body. Researchers who work in the medical field have been attempting to use animals and plants to extract biocompatible materials with minimum toxicity and high efficiency, which could be applied for human disease diagnosis and therapy. These materials must have human tissue characteristics such as matched hydrophilicity and porosity for providing a microenvironment for suitable cell proliferation and food/waste/drug exchanges. [1][2][3] Since almost two-third of the human body consists of chameleons adhesives, designing and applying biocompatible materials to firmly adhere to wet surfaces and be capable of holding a significant amount of water in their networks, preserving high mechanical properties against burst and purge pressure, and showing the ability to facile functionalization is in the center has become of particular attention. 4 Firm adhesion to the wet surfaces along with to severe wave conditions by blue mussels has been an inspiration for the development of novel, sophisticated wet adhesives. Since the degree of interaction between wet adhesive and tissue is the most key factor controlling and determining adhesion efficicncy (particularly when repairing tissue is the target), dealing with chemistry of such adhesives is of critical importance in fabrication of bio-based adhesives. The mussel chemistry sheds light on the mechanisms of adhesion and helps scientists to design better hydrogels and complexes for biomedical applications. [5][6][7] 2 | MUSSEL INSPIRED CHEMISTRY Mytilidae mussels have inspired scientists to find a proper attachment in wet conditions, which usually is a complicated process. Blue mussels (Mytilus edulis) attach to rocks strong enough such that wind and waves are quite often unable to detach them. This wet adhesion phenomenon has attracted much interest, and numerous investigations have been devoted to reveal the mechanisms and chemicals responsible for this natural adhesion process. 8,9 The mussel's foot is similar to chemical plants that produce constructing materials for manufacturing byssus (i.e., a bundle of hundreds of threads) (Figure 1a). The raw materials are produced within three glands (i.e., phenol, collagen, and accessory glands) in the foot and are delivered through a microfluidic channel known as a ventral groove in which byssal threads are made in a process similar to injection molding of polymers. The foot, like a chemical reactor, provides controlled conditions for the raw materials to react at a defined ratio and makes the core, cuticle, and plaque. In adult mussels, each byssal thread is approximately 2-6 cm in length, composed of three parts of different morphology: (1) adhesive plaque, (2) distal, and (3) proximal sections (Figure 1b). 10 The elastic proximal section, which is attached to the foot and spread from the stem, is made of loosely packed collagen proteins with a helical structure endowing threads with elasticity and load-bearing properties. 10 On the other hand, the distal section is primarily made of tightly packed aligned collagen fibrils (i.e., prepolymerized collagen or Pre-Cols), which provide threads with three kinds of critical flanking regions; silk (truly rigid, 10 GPa), elastin (gentle, 2 GPa), and polyglycine (amorphous and intermediate; Figure 1c). 11 Such a structure provides a gradual movement from the soft tissue of the foot to the hard substrates (i.e., rocks) at the spatulate plaques. 12,13 The adhesive plaque, which originates mainly from phenol glands, is the first protein that is injected, followed by the thread core (from collagen glands) and, finally, the cuticle layer (from accessory glands). Cuticle, a 2-5 micron thick layer with granular morphology (Figure 1d), is primarily composed of Mfp-1, which contains approximately 1 wt% metal ions resulting in high stiffness (2 GPa, compared to underlying collagen fibrils with $0.5 GPa) and hardness (0.1 GPa). It is worth noting that elongation at break for byssal threads is around 120%. 14 The adhesive plaque has a porous structure where its voids pattern depends on habitat conditions. This structure is proven to reduce crack propagation and enhance plaque ductility. 16 However, plaque's chemical structure is the most prominent factor in plaque adhesion.
Mussel adhesive proteins (MAPs) secreted from blue mussels, known as mussel foot proteins (Mfp), are responsible for firmly adhering mussels to various surfaces under wet conditions. Six different types of MAP proteins have been identified (Table 1). 17,18 These proteins are highly cationic (i.e., the MAPs are polyelectrolytes) and have a high gather proteins in their fluid-controlled state. This separated phase will not dissolve easily in aqueous media and shows poor interfacial tension in seawater. 10 The term coacervation refers to an electrostatically induced liquid-liquid phase separation, for example, when two oppositely charged polyelectrolytes are mixed. All MAPs are positively charged with a high isoelectric point (pI). 29 However, Mfp-3S is the unique MAP that can self-coacervate ( Figure 2). 10,17 Under the physiochemical conditions inside the mussel foot cavity, colloidal suspensions of Mfp-3S undergo a liquid-liquid phase separation, which results in coacervate formation. 30 This self-coacervation process also has a key role in certain biological systems like squid beak formation. 30 It seems that electrostatic and hydrophobic interactions between nonpolar Mfp-3S polymers are responsible for phase separation. 29 Coacervation of MAPs is believed to be the method used by mussels for their initial adhesion to wet surfaces. 29 The immediate adhesion mechanism of the mold is characterized by a close correlation connecting the coacervate's easy secretion/surface wetting characteristics and the primordial interface sticky function of the Mfp-5 and Mfp-3 as surface MAPs, effectively contacting aquatic surfaces. 11,29 These metastable fluidic coacervates can further undergo phase inversion, crosslinking, and solidification. All in all, the adhesive protein coacervation operation includes several steps. At first, the negative isoelectric point of proteins and positive charges aggregate at acidic region (pH $ 5); then, the neutralization of opposite charges T A B L E 1 General chemical and physical characteristics of mussel adhesive proteins which form the structure of the adhesive plaque followed by the secretion of other proteins that make threads as discussed earlier.
Note that Mfp-3s, due to possessing higher proportion of hydrophobic amino acid residues compared to Mfp-3f is capable of inhibiting DOPA from oxidation, especially at pH >7. 33 It was observed that DOPA moieties have a significantly higher oxidizing ability in Mfp-3s compared to Mfp-3f, that is, DOPA is less suspicious of oxidation in the presence of Mfp-3s, which result in improved adhesion in neutral to basic environments. It is believed that this phenomenon is related to hydrophobic interactions of Mfp-3s, which results in the creation of a hydrophobic microenvironment encompassing DOPA residues and protecting them from the surrounding environment. The whole process for byssal threads formation takes about 5 min. After 8-12 days of this process, the plaque's adhesive strength increases 100% due to seawater exposure. 34 This postprocessing phenomenon depends on physiochemical conditions of seawater such as oxygen concentration and pH ( Figure 3). 35 Figure 4 indicates various byssal positions when mussels faced different external forces.
From this perspective, nature using biological processes based on chemical functionality such as dopamine and eugenol inspires scientists to design a material based on mussel chemistry for various uses such as adhesive for biological media. Catechol is the central part of such molecules responsible for adhesiveness. In this regard, we will F I G U R E 3 Mussel adhesion to the surface at various pH. (a-1) at pH < 7, MAPs readily transfer to the surface; Lysine and DOPA facilitate the development of bident H bonds and surface oxide coordination interactions. (a-2) DOPA's auto-oxidation at the basic region (pH = 7.5-8.2) is an issue that could cause adhesion to reduce by more than 75% or 95% in comparison to low pH such as 3 and 5, respectively. Along with the formation of dopaquinone, catechol oxidase, and redox transfer between DOPA and iron (III) ions, trigger the formation of crosslinks in the plaque 34,35 F I G U R E 4 Different positions of threads in various situations describe the mussel-inspired chemistry to pave the way to design appropriate substrates for biomedical applications. Catecholamines are a set of chemical neurotransmitters that contribute to regulating physiological processes while also contributing to several diseases, including cardiovascular, neurological, and endocrine diseases. 38 Tyrosine and DOPA are the main precursors for manufacturing catecholamines in living organisms. 39 Dopamine is a neurotransmitter that is very important in the brain and the human blood plasma ( Figure 5). Besides, it is a precursor of norepinephrine and epinephrine hormones. 40 Unbalanced dopamine levels lead to various disorders and illnesses like Parkinson's disease. 41 Dopamine is found in the central nervous system (CNS), where its axon terminal concentration is maximum for all body parts. 42 3,4-Dihydroxy-catechols are predominantly found in dopamine and its derivatives, such as DOPA, while 2,3-dihydroxy-catechols (especially 2,3-dihydroxybenzoic acid) are found in siderophores secreted by microorganisms, which strongly chelate Fe 3+ when transporting it through the cell membrane. 44 Enterobactin is the siderophore of high affinity found primarily in Gram-negative bacteria such as Salmonella typhimurium. The primary function of this siderophore is acquiring iron, as shown in the following figure. Besides animal species, catechol structure can be observed in plant species such as natural dyes like flavonoids (e.g., quercetin), which results in antioxidant and strong radical scavenging potential. 45,46 Tannic acid, which is a polyphenolic compound, has been recently at the center of attention in biomedical applications. 47,48 This weak acid rich in pyrogallol/ catechol functional groups endow the acid with antioxidant, radical scavenging, and adhesive properties. 49,50 Caffeic acid is another catechol-containing carboxylic acid, which is a precursor to the F I G U R E 5 Chemical structures of catechols found in human plasma. Cys 5-S-cysteinyl, DOPAC 3,4-dihydroxy phenylacetic acid, DHPG 3, and 4-dihydroxy phenyl glycol 43 synthesis of lignin in plants. 51 This natural polyphenol is also found in coffee and certain fruits, oils, and herbs and possesses antioxidant properties. 52 On the other hand, different catechol-containing derivatives such as dopamine can be manufactured through substitution on the benzene ring or the side chain. 53 These substituted functional groups affect the charge distribution on the benzene ring, which alters the reactivity and adhesion properties of catechol groups. Besides, acrylamide derivatives such as N-(3,4-dihydroxyphenethyl)methacrylamide have also been used in manufacturing MI polymers. 54 These derivatives, which have been intensely used in polymer synthesis, would be studied in more detail.

| CATECHOL CONTAINING MATERIALS
The vicinal hydroxyl groups (catechols) of DOPA are a critical factor in its wet adhesion. The maximum adhesion of Mfp-3 and Mfp-5 F I G U R E 6 Different interactions between catechol and other moieties occurs when pH is less than 3. The adhesion strength significantly diminishes at pH = 5.5 and completely disappears at neutral pH. 25,55 Thanks to the redox properties of catechol, DOPA serves not only in surface adhesion but also in cohesive forces within the bulk of the adhesive plaque.

| CATECHOL INTERACTIONS
The catechol group consists of two adjacent hydroxyl groups attached to an aromatic ring. There are π-electrons systems below and above the benzene ring, resulting in a quadrupole load distribution and thus allowing electrostatic interactions. In other words, this electron-rich system can interact with various species, including cations, anions, neutral metals, and other systems like aromatics. Some of these possible interactions are metal-π interactions, polar molecules-π interactions, aromatic-aromatic interactions (π stacking), donor-acceptor interactions, anion-π interactions, cation-π interactions, C-H-π interactions. It is expected that catechol groups which are composed of two neighboring hydroxyls attached to a benzene ring experience similar interactions. 56 Besides, OH groups would endow their new interactions with other species. strong π-π interactions with aromatic ring-containing platforms (e.g., polystyrene). 56,58 Also, catechols, through an attraction between their benzene rings and cations (cation-π interactions), are capable of adhering to cation-containing surfaces (Figure 6c). 57 Figure 6d illustrated that catechols play a role as a robust anchor to modify and functionalize the surface of diverse metal oxides (silver, gold, silicon, and titanium oxides) to nickel-titanium alloys via the reversible interfacial bonds. 59 Figure 6e explains that the chelation of various metal cations by catechols is suitable to fabricate stable yet reversible complexes, which have many applications in the synthesis of pHresponsive drug delivery systems, hydrogels with self-healing features, and soft actuators. [60][61][62] Also, the reaction between catechols and the boronic acids results in the creation of dynamic covalent bonds between oxygen and boron atoms (Figure 6f). 63 Catechol can be oxidized in the presence of oxidation agents (e.g., sodium periodate) to produce a radical form of an ortho-quinone structure ( Figure 6g). 57,64 In addition, catechol oxidase can catalyze the reaction between catechol derivatives and oxygen. In mildly basic aqueous solutions or even in the air, catechols are capable of oxidizing spontaneously. Figure 6h exhibits the fact that dimers were formed whenever quinone mole- with boronic acids. 78 Boronate-catechol complexes can prevent DOPA from oxidation. 79 Thus, these complexes have been used as a temporary protecting group when modifying monomers or polymers with catechol. 80

| CHEMICAL INTERACTIONS
Metallic cations, especially with high charge density, can interact with the electron-rich region below/above benzene rings. Electrondonating groups (e.g., NH 2 , -OH) strengthen this interaction while electron-withdrawing groups (e.g., CN, NO 2 ) weaken it.

| Oxidation
Catechol can be oxidized in the presence of oxidation agents (e.g., sodium periodate, ammonium persulfate, hydrogen peroxide, mushroom tyrosinase, and horseradish peroxidase) into their quinone form. For example, catechol oxidase can catalyze the reaction between catechol derivatives and oxygen. In aqueous, mildly basic solutions, catechol is prone to auto-oxidation ( Figure 7a). Besides, as shown in Figure 7b catechol also spontaneously oxidizes when exposed to air or water. 81 In all cases, the oxidation is accompanied by a clear "catechol tanning"-the color turns dark red to almost black. 63 Quinones are highly reactive intermediates that can be covalently attached to the nucleophilic functional groups (e.g., thiol, thiol acids, primary and secondary amines, cyanamide, or imidazole) via nucleophilic addition reactions (Michael addition or Schiff base substitution as shown in Figure 8). 83 The reaction of quinones with water yields a 1,4-addition product which is an unstable intermediate. 39 Besides, quinones can react with alcohol functional groups through Michael-1,-4-addition. The reaction between quinones and carboxyl functional groups yields esters. 39 Furthermore, catecholamine derivatives could be easily oxidized. 39 Catecholamines (except norepinephrine) oxidize to form semi-quinone and eventually to quinone form. Quinones reactivity is essential in biological systems. Moreover, a controlled condition in the dopamine microenvironment is essential for adequate brain function. 38 The oxidation of dopamine may result in Parkinson's disease. 86 dopamine is a qualified antioxidant that acts as a free radical scavenger protecting neurocytes from oxidative stress. 38 Auto-oxidation of DOPA at neutral pH is problematic, which can result in more than a 95% decrease in adhesion. DOPA is converted to dopaquinone under oxidizing conditions such as chemical or elevated pH. Nonoxidized DOPA contributes to adhesion, while oxidized DOPA mainly contributes to cohesion. 87 Oxidation of N-acetyl dopamine and N-β-alanyl dopamine usually occurs during cuticular sclerotization (Figure 9). 88 The functionalization of the catechol with electron-withdrawing groups increases the oxidation rate with oxygen, while electron-donating groups decrease the oxidation rate.

| Nucleophilic interactions
Oxidation intermediates like quinone and quinone methide may react with nucleophilic groups like thiol and amines. The reaction of amine and catechol is critical in many biological systems, including mussel adhesive proteins, polymerization of specific protein subunits to create cytoskeleton in insects, and the synthesis of melanin pigments. 89 As discussed earlier, along with DOPA, the L-lysine amino acid is found extensively in blue-mussel Mfps. The primary amine groups in L-lysine may undergo reaction with o-quinone moieties by Schiff-type reaction, which helps to solidify secreted proteins in blue mussels. 89 The beak material in jumbo squids is a mineral-free sclerotized chitinous (the protective outer layer of some species of insects and crustaceans) composite in which covalent crosslinking between DOPA and histidine amino acid is observed in the form of multimers 90 ( Figure 10). Histidine is an essential amino acid that can change into histamine upon decarboxylation. Amine-catechol chemistry, which is common in biological systems, has focused on References 91 and 92.
The reaction between catechol and thiol is found in many biological systems. 31 Orthoquinones from the oxidation of catecholamines can react with thiol-containing cysteine resulting in neurotoxic cysteinyl catecholamine. 93,94

| Oligomerization
The

| Polymerization
Autoxidative dopamine polymerization usually occurs in basic media under oxygen presence. 96 However, it has recently been reported that dopamine could be polymerized under acidic conditions (pH <5.5). 97 Plasma-activated water (PAW), prepared through a micro hollow cathode discharge device, was used as a polymerization medium. The obtained acidic PDA has a similar chemistry to the basic PDA, which is routinely synthesized in basic conditions, but the particles of acidic PDA show superior stability at different pH conditions. This PAW-PDA seems important for large biomedical applications.
DOPA residues can undergo polymerization reactions while forming oligomers with up to six attached monomers, which is responsible for the rapid curing of catechol-containing adhesives. 92

| Crosslinking
Crosslinking and reactivity of catechols strongly depend on the pH. In mild acidic environments (i.e., pH = 5.7-6.7), quinone methide, which is a highly reactive intermediate, becomes more stable and retard further reactions resulting in a slower rate of crosslinking. 98 Under neutral to mild basic conditions (pH = 7.4-8), the rate of crosslinking reaction is high. In this range, quinone directly transforms to α,β-dehydrodopamine, which may react with dopamine quinone, O 2 , or oxidization agent to yield its quinone form. At higher pH values, dicatechol species have been observed. Aryloxy radicals, which are generated by both catechol and quinine moieties, are responsible for the fast cross-linking of catechol-modified polymers. 99 In quinone methide ( Figure 12), there is one carbonyl oxygen substituted on the benzene ring. This makes it more polar and extremely reactive compared to quinone with two carbonyl oxygen. Para-quinone methides are related to catecholamines. F I G U R E 8 Schematic of Michael addition and Schiff base reaction can occur between quinone and primary amines 84,85 derivatives as side chains or end-caps. On the other hand, monomers or oligomers may also be functionalized with catecholic derivatives before polymerization to yield copolymers. Besides, catechol-modified initiators may also be used to create catechol-containing polymers. 39

| Premodification strategies
As discussed in the introduction section, the mussel's foot should provide a cavity of controlled condition before the secretion of MAPs. Adjusting the pH, oxygen concentration, and ionic strength are prerequisites for successful adhesion. In this regard, adjusting and controlling the conditions of the reaction medium is very important when designing and synthesizing catechol functionalized polymers and monomers or other catecholic compounds. Besides, several methods may be utilized to protect OH or amine in catechol groups during functionalization or polymerization. Protecting the α-amino groups is critical in peptide chemistry. These protecting groups should be removed efficiently and fast while yielding easily removable byproducts. 100 9-Fluorenyl methoxycarbonyl (Fmoc) and tert-butyloxycarbonyl (Boc) group are the most frequently used protecting groups for α-amino in peptide synthesis which are used in Fmoc/tert-butyl (tBu) and Boc/benzyl (Bn) methods, respectively. In organic chemistry, 9-fluorenylmethoxycarbonyl (Fmoc) is a baselabile protecting group that is used for protecting α-amino groups. 101 In this regard, several protecting groups have been utilized to protect amino groups (e.g., Fmoc, Boc, carbobenzyloxy, acetyl, benzoyl, benzyl, and carbamate), the carboxylic acid groups (methyl esters, benzyl esters, tert-butyl esters, and silyl esters), and phenolic hydroxyl groups (e.g., tert-butyl ether, methoxymethyl, tetrahydropyranyl) after a modification with catechol. Acetyl, 102 t-butyldimethylsilyl chlorides (TBDMS-Cl), 103 cyclic ethyl orthoformate (Ceof), 101 carboxybenzyl, 104 acetonide, 105,106 methyl ether 107 have been used as protecting groups for catechol during multistep organic synthesis methods.
Diethers or diesters can be used to protect catechol similar to strategies that are used to protect phenols from yielding cyclic esters, and cyclic acetals and ketals. 108 Furthermore, several strategies (e.g., by using acids for Boc or bases for Fmoc) have been developed to remove these protecting groups after synthesis. 100

| Natural polymers
Natural polymers can be classified into polysaccharides, proteins, and polyesters. A beak mimic (which is based on DOPA) has resulted in manufacturing water processable chitosan composites. 109 α-Amino acid N-carboxy anhydrides (NCAs) are reactive derivatives of amino acids that could be prepared through phosgene treatment. 104 Polypeptides can be prepared through ring-opening polymerization of NCAs. Copolymerization of NCA monomers of L-lysine and L-DOPA yields water-soluble copolymers. After crosslinking with oxidants, these copolymers create adhesives that are resistant to moisture and show high adhesion to steel, glass, and synthetic polymers. 104 Pluronic L-31 will serve as an initiator of NCAs ringopening polymerization to yield thermosensitive hyperbranched poly(amino acid). 110 However, there are a few research works on the functionalization of proteins with catechol, gelatin, 111 silk fibroin, 112 and collagen. 18 Polysaccharides such as alginate, 113 chitosan, 114 hyaluronic acid, 115 dextran, 116 chondroitin sulfate, 117 and cellulose, 118 have also been modified using catechol chemistry. Since these are water-soluble polymers, the catechol modification is usually carried out using EDC/NHS chemistry to create side chains with catechol functionality Raper-Mason pathway for the biosynthesis of melanin using -NH 2 , OH, and COOH groups on the sidechains of polymers.
Alginate is one of the most studied polysaccharides in biomedical applications. Unmodified alginate can immediately change into a gel in the presence of Ca 2+ or other divalent cations because of ionic crosslinking (electrostatic interactions) with the carboxylic groups. 119 This phenomenon is fundamental for the encapsulation of drugs and cells.
However, because of physical cross-linking, the dissociation of ionic bonds between cations and carboxylic acid groups on the alginate is facile, resulting in ion dissolution. 119 On the other hand, chemical crosslinking increases the physical stability of the gel, while chemical coupling agents can damage cells and proteins when encapsulating. 120 Accordingly, a combined approach has been used in which cell encapsulation is triggered by an ionic coupling agent followed by a gradual substitution by covalent crosslinking. 121 The gels' stability and at a similar rate. Thus, gel maintains its integrity in contrast to common alginate hydrogels, which can "dissolve" in excess of water. The relative swelling for alginate-catechol gels was measured to be 660% compared to 350% for ordinary alginate gels. 121

| Synthetic polymers
Catechol may be used to manufacture copolymers, or it may be grafted to synthetic polymers. Besides, it may be used to synthesize end-capped polymers.
DOPA was also used for various copolymers. Stable and biocompatible DOPA and lactide block and graft copolymers (i.e., PDOPA-g-PLA and PDOPA-b-PLA) can be generated using lactide ring-opening polymerization in the presence of PDOPA, which also acts as an initiator. 122 The LA:DOPA molar ratio, determine the molecular weight and composition of the obtained copolymers. It was observed that the thermal stability and degradation rate of graft copolymers are higher The T g of the polymer is 106 C, while its melting point is surprisingly higher than 200 C. the molecular weight was reported to be around 12,000 g/mol. High degradability and excellent biocompatibility were also observed. Cell adhesion of PDOPA was comparable to that of tissue culture polystyrene (TCP). In this study, pyrene and PLA have also been grafted to PDOPA. Reactive amine groups on the PDOPA enable its functionalization with other chemicals and polymers. Poly(ethylene) glycol (PEG-g-catechol) (with grafted catechol) can be prepared through polymerization of PEG in the presence of dopamine. 124 The obtained copolymer can be used for the PEGylation of different surfaces and manufacturing antifouling coatings. Catechol acetonide glycidyl ether (CAGE) is a protected catechol derivative that has been utilized to endow catechol functionalities to hydrophilic polyethers. 125

| Zwitterions
Zwitterions are organic salts containing two or more functional groups, containing at least one positive and at least one negatively charged functional group. 137 They have been used in biomedical applications. 138 More interestingly, phospholipid as a major comprising component of cell membranes can be considered a zwitterionic derivative. The hydrophilic head of phospholipids is made of a covalently bonded pair of the cation (cholinium) and anion (phosphonate; Figure 14). 139 Copolymers of zwitterionic and dopamine have been used for multifunctional coatings. 141  The former functionality, thanks to catechol chemistry, contributes both to the attachment of microgels (i.e., like an intraparticle crosslinker) and adhering to diverse surfaces. The latter (i.e., zwitterion) enhances the water absorption capability on the surface, which results in antifouling and anti-fogging characteristics. 141 Wet adhesives based on catechol chemistry may be limited due to strictly controlled steps and complex and costly chemicals used properties of them. 157 Protein corona is a layer of proteins adsorbed onto various surfaces such as particles which results in disadvantages such as masking surface properties of nanoparticles (i.e., the chemical or biological functionalities that were deliberately imparted to nanoparticles) and providing nanoparticles with a biological identity that is detectable by the immune system. 158 PEG coating has been utilized to control protein corona and cellular uptake of nanoparticles. [159][160][161] Interestingly, the type of proteins that adsorb on PDA-coated gold nanoparticles depends on the dopamine concentration. 144 The diffusion of nanoparticles into the cells, known as cellular uptake, is critical in diagnostic and therapeutic applications. Besides, their fate in the physiological environment and the intracellular condition is of prime importance to do their functions effectively. 157,162 For However, it has limited osteoconduction and osteoinduction capacities. 164 The surface of Ti can also be modified by PDA-coated Fe 3 O 4 nanoparticles to enhance osteogenesis. 165 PDA-coated Ti implants, with enhanced corrosion resistance, higher cell viability, and lower contact angle was used in dental Implants applications. 166 Besides, the integration of hard medical implants (e.g., Ti) with soft tissue and the wound healing process can be improved through a layer of gelatin hydrogel on a PDA-coated implant. 145 Silicon, due to its biocompatibility, low surface energy, smooth surface, and chemical, thermal, and biological stability, has been used in biomedical applications such as bio-implants, coating for cardiac pacemakers, shunts, and microfluidic devices. 167 However, there are still concerns, especially adverse immunological reactions, about the long-term utilization of silicon-based implants in the body. 168,169 Coating the implants with various biomolecules can reduce these concerns. Biocompatible PDA coating can adhere to the PDMS surface, increasing cell attachment, proliferation, and differentiation. Besides, the hemocompatibility of silicone-based implants can be enhanced using PDA and hyaluronic acid (HA) coatings. 170 Moreover, PDMSbased organ-on-a-chip devices can be more effectively designed using PDA coating, which can improve cell metabolism. 171 Besides, polymeric scaffolds have been coated with PDA to improve its cellular interactions and immobilization of biomolecules. 172,173 This will be covered in the section on biomedical applications. PDA has been used as a general approach for bio-surface modification.

| BEYOND CATECHOL CHEMISTRY
Electrophilic substitution reaction with chlorine in para position results in stronger adhesion. This is observed in cement proteins secreted by sandcastle worms in which there is a remarkable amount of chlorinated derivative of DOPA. 174 Such electronwithdrawing groups, reduce the dissociation constants (pK a ) of hydroxyl groups of the phenolic ring (which promotes catecholmetal complex at lower pH and higher stoichiometry) and decrease their redox potential which makes catechol oxidation harder. 175 Similarly, substitution with the nitro group also enhances the adhesion and reactivity of catechol while improving the thermal and oxidation stability of catechol. [175][176][177][178][179] Nitrodopamine can quickly cure and attach to biological surfaces in acidic pH (compared to dopamine), making it a promising candidate for manufacturing bioadhesives for acidic tissues. 176 Increased degradation rate, degradation by light, and reducing pK a are other properties induced by nitro substitution. [178][179][180] Adding one extra OH group to the benzene ring makes trihydroxybenzenes, which has a remarkably enhanced tendency toward complexation with metal ions and boronic acid as well. 181,182 As discussed earlier, this characteristic is utilized by some bacteria siderophores for iron acquisition in scarce environments. Pyrogallol or gallol is a trihydroxy benzene with three vicinal hydroxyl groups on the benzene ring ( Figure 16). In other words, this organic compound has one more OH relative to cate-  designed and prepared a bioceramic scaffold using a 3D printing technique for bone cancer treatment and tissue regeneration. By taking into account the benefits of using biocompatible PDA with outstanding biodegradability and its elevated photothermal capability, they prepared uniform PDA nanolayers via its selfassembling process onto an as-prepared 3D bioceramic (Nagel) scaffold. This scaffold provided an exquisite environment for osteoblastic stem cells of rabbit bone to attach, differentiate, and proliferate. According to this report, the regeneration of bone tissue was highly accelerated and desired, whereas photothermal therapy was in progress. 201

| Biointerfaces
In several applications, such as neural prostheses, neurons on a chip, and tissue engineering, the interfacial contact between neuron cells and other surfaces is inevitable, which results in inflammatory responses. [202][203][204] Consequently, surface modification strategies (e.g., immobilization of biomolecules on the surface) have been utilized to decrease such responses. 201 However, several metals (e.g., gold, stainless steel, platinum, titanium, tungsten) and insulating materials (e.g., parylene, polyimides, SU-8, iridium oxide, and titanium Polydopamine can increase the hydrophilicity of materials via its various hydration mechanism ( Figure 19). 216

| Hydrogels
High-water polymeric hydrogels and tissue-imitating mechanical properties were used as cell attachment, growth, and delivery scaffolds.
They also have numerous biomedical implementations, including drug/gene delivery, wound management, soft contact lenses, tissue engineering, and hygiene products. 220 Until now, different kinds of natural and synthetic hydrophilic polymers were used to make hydrogels. Stimulus-responsive hydrogels belong to a wide range of smart hydrogels that can be adjusted using different stimuli such as temperature, pH, and light. 221 F I G U R E 1 9 A different mechanism of (a) catechol group reaction, (b) Polydopamine hydration 216 Injectable hydrogels that could be efficiently delivered to the target site using a syringe are very interesting for minimally invasive therapies. 222 The injectable hydrogels may also carry cells, genes, or theranostic agents. When injected into the body, they experience to MFP-3s in blue mussels in which high content of hydrophobic amino acid residues creates a local microenvironment and protects DOPA moieties from oxidation, as discussed in Section 1.
Tannic acid discussed earlier is rich in catechol and gallol functional groups. Abundant hydroxyl groups enable it to make a hydrogen bond with hydrogen-bond donor polymers such as polyvinylpyrrolidone (PVP). Physically crosslinked reversible hydrogels can therefore be designed, showing pH-dependent switching. In other words, as the oxidation of catechol to quinone depends on pH, hydrogen bonds forming and dissociating are also pH-dependent, resulting in a reversible and dynamic hydrogel. According to this idea, pH-responsive hydrogels were manufactured using tannic acid PVP, which exhibits shear-thinning, self-healing, and quick self-recovery properties, which are prerequisites for injectable hydrogels. 225 Furthermore, catechol and gallol functionalities enable metal complexation such that these F I G U R E 2 0 Representation of preparation of the composition of ε-poly-L-lysine and catechol and its application for tissue engineering hydrogels can be further cross-linked using Fe 3+ ions, resulting in a dual-responsive reversible hydrogel. Avoiding chemical crosslinkers indicates these hydrogels' high biocompatibility for applications in various biomedical fields.
Chitosan is a mucoadhesive polymer that can be used for drug delivery, including Buccal mucoadhesive systems. 226 In addition, MI chemistry can enhance the mucoadhesiveness of chitosan and its derivatives. In this regard, the mucoadhesive hydrogel was fabricated based on chitosan modified with hydrocaffeic acid as catecholcontaining chemicals. 114 Besides, genipin was used as the crosslinker, which is a naturally occurring aglycone with low cytotoxicity. 227 Contrary to neat CS hydrogels, which are released from the porcine mucosal membrane only after 1.5 h, the catechol-functionalized CS exhibited long-lasting adhesion. CS-cat hydrogels were successfully used in a sustained buccal drug delivery system on the rabbit model. 114  Hemorrhaging (bleeding) is one of the significant problems associated with blood leakage from blood vessels. Massive hemorrhage can cause serious health problems and even death. Thus, preventing blood loss is very important in these cases. A process that causes the bleeding to discontinue is known as Hemostasis, which is considered the first step in the wound healing process. The various hemostatic agent has been developed based on a different mechanism of action which are commercially available. 233 Blocking the blood flow through strong adhesion to the tissue surface is the main mechanism of action of some hemostatic agents. However, strong adhesion under the wet condition of bleeding can be quite challenging. The MI chemistry has been considered as a potential for this problem. Catechol-modified chitosan has been used in manufacturing hemostatic swabs to stop bleeding. 234 PDA coated silica nanoparticle was also developed for bleeding control. 235  PNIAPAM-polydopamine nanoparticles hydrogels were also applied to wound healing. 241 These thermosensitive hydrogels show improved cell affinity, self-healing properties, and tissue adhesive properties, and they can immobilize epidermal growth factors as well.

| Wound management
When imposing NIR radiation, the composite hydrogels show volume contractions, which indicate a photothermal phase transition, while similar behavior was not observed for pristine PNIPAM hydrogels.
This contraction was used in drug delivery too.
Healing chronic wounds like pressure sores, diabetic foot ulcers, chronic venous ulcers, and arterial insufficiency are very difficult. 242 The healing of chronic wounds is inhibited because of constant agitation of the inflammatory system induced by protease enzyme, reactive oxygen species (ROS), and exudates. Diabetic foot ulcers are the most common chronic wounds which impose a high cost on the healthcare system of countries. Approximately two-thirds of amputations are in diabetic patients. 243 The high concentration of protease can degrade different species such as elastin, collagen, and growth factors while ROS oxidizes biomolecules followed by triggering the inflammatory system. It was reported that naturally derived phenolic compounds show antioxidant properties. 244 This antioxidant behavior was also observed for different catecholamines. Thus, catechol-containing chemicals may also show antioxidant behavior along with wet adhesion.
Hydrocaffeic acid (HCA) grafted to random copolymers of N-vinyl caprolactam (V) and 2-hydroxyethyl methacrylate was used as a bioadhesive resorbable membrane for chronic wound healing.
Catechol-containing HCA side chains provide wet adhesion for the copolymer. The biocompatible membrane shows an anti-inflammatory effect and ultraviolet screening properties as well. 198 Conjugated N-vinyl caprolactam (V) and 2-hydroxyethyl methacrylate (H) with catechol to produce a biocompatible tissue adhesive for wound healing. 198 The scar is a fibrous connective tissue that may replace the healthy tissues after the wound heals. In scar tissue, small collagen bundles are aligned parallel to each other while normal tissue is composed of collagens with a random basketweave structure. 245  Scar management after severe burn injuries has attracted much attention. Stem cell-based therapies, pharmacological methods, and surgical methods have been utilized for burn scar management. 248 Catechol chemistry can also be used to burn injuries. As stated above, catechol moieties depend on environmental pH. This enables pH-responsive drug delivery system design.

| Drug/gene/cell delivery
PD-coated PCL nanofibers were used for pH-triggered drug delivery systems, where positively charged molecules release faster in acidic media. 252 Thiol-catechol chemistry has been utilized to manufacture pHresponsive mussel-inspired polydopamine capsules for cancer therapy.
The CDDS contains an anticancer drug that releases endosomal/ lysosomal pH in the cells while its release rate is very slow at physiological pH. pH-responsive Dox-loaded protein nanoparticles were manufactured using Iron(III)-DOPA complexes. 253 Doxorubicin release is mainly controlled by the structural alternation of the Iron (III)-DOPA complexes caused by acidic pH. In vitro assays validated that using the Iron(III)-DOPA complexes due to the cellular uptake efficiency and excellent cytosolic release, showed wonderful cytotoxicity toward tumor cells. Mucoadhesive MI hydrogels were also used to deliver drugs for application areas like mucin and buccal. 229,230 Extended localized effects of medication, prevention of gastrointestinal drug metabolism, and high patient compliance are listed as the advantages of using buccal mucosa for drug delivery systems. Suhair Sunoqrot et al. 229 to achieve an enhanced gastro-retentive oral drug delivery system, designed a mussel-inspired chemistry/methoxy poly(ethylene oxide)-block-poly(ε-caprolactone) (PEO-b-PCL) copolymer having different mass ratios and evaluated them through in vitro tests. In another study, chitosan, as a subclass of mucoadhesive hydrogels, was functionalized using MI chemistry, then the polymer was crosslinked using genipin to be utilized as a novel drug carrier.

| Therapeutic agents and cancer therapy
Application of the mussel-inspire strategy to fabricate super materials for photothermal therapeutic (PTT) uses (e.g., cancer therapy, selfhealing hydrogels), has recently drawn increasing interest. According to the latest news of the World Cancer Report, the population who are suffering from cancer will increase to 15 million cases by 2020, so cancer therapy using photothermal methodology has become so important. Diagnostic (e.g., contrast agents) and therapeutic agents (e.g., photothermal therapy agents) coating with PDA were discussed earlier.
Here, we emphasize catechol-based chemicals which are directly used as therapeutic agents. PDA can absorb NIR radiation.
Radiotherapy and chemotherapy strategies are standard cancer treatment methods that can cause systemic cytotoxicity. However, as a minimally invasive method and with minor side effects, phototherapy (PTT) has attracted much attention. 261 In combined photodynamic (PDT) and photothermal (PTT) therapy, hydrophilic PDA nanoparticles can be used as drug nanocarriers to kill bladder cancer cells. 262 The therapeutic agent consists of a hydrophobic photosensitizer drug (chlorine e6, Ce6) dispersed through interaction with PDA nanoparticles. The interaction between PDA and drug is proposed as the mechanism for long-lasting drug release. It was observed that cell uptake of PDA nanoparticles remarkably increases when Ce6 is incorporated.
After irradiation with 665 nm red light, more effective therapy was observed for combined therapy. This strategy was proposed as a F I G U R E 2 1 Various types of PDA-coated materials for antimicrobial applications 146,257 general method for mucosal drug delivery (such as intestine and bladder) in which much penetration of hydrophobic photosensitizer is very limited.
Combined PDT, PTT, and chemotherapy using multifunctional therapeutic agents have also been used for cancer treatments. 263 Magnetic Fe 3 O 4 coated nanoparticles with a PDA-layer and surrounding PEG corona were used for cancer treatment applications Implanted devices such as electrodes could be manufactured using MI chemistry. 273 A general scheme for biofunctionalization of the neural interface was proposed by Kang et al. 207 The most important advantage of MI conductive hydrogels for application in implantable devices is that they can tolerate both wet conditions and temperature fluctuations. 274 9.11 | Electronic skin/wearable electronic Human skin includes a network of different sensors, which obtain various environmental information and send them to the brain. The information is in particular concerned with thermal and tactile stimuli. An ambitious goal is to manufacture an electronic skin (named E-skin) inspired by human skin, which means to develop a network of sensing elements embedded in tough, high stretchable, skin adhering, and biocompatible materials with good cell affinity. More ambitious goals are to supply these artificial skins with more functionalities such as chemical sensing, diagnostic and monitoring capabilities, odor, and taste sensing (electronic nose and tongue). 275 More efficient robots can be designed through the application of these smart skins. Self-healing and adhesion to human skin capabilities are essential for E-skins. Catechol moieties, as discussed earlier, show self-healing capabilities.
High stretching also can be achieved using catechol-containing hydrogels. These properties have made them attractive materials for developing electronic skin and wearable electronic applications. For example, a strain sensor with high sensitivity has been manufactured using polyacrylamide hydrogel and dopamine-modified talc nanoflakes. 276 This hydrogel shows enhanced cell affinity compared to pure PAAM.
For electronic applications such as E-skins, electrical conductivity is an essential property. 277 In this regard, conductive species, such as carbon nanomaterials and conducting polymers, can be included in catechol-modified hydrogels. A sensor for monitoring human motion was designed through the incorporation of single-wall carbon nanotubes in polyvinyl alcohol and polydopamine hydrogel. 278 The hydrogel shows self-healing and skin adhesiveness while it creates no cytotoxicity.
Conductive hydrogels based on dopamine-modified reduced graphene oxide and PAAM can be used in motion monitoring and as an electromyography (EMG) electrode, as well. 273 The hydrogel was used as an implanted EMG electrode. Furthermore, it was observed that the conductive hydrogel improves the proliferation and adhesion of the stem-cell of the bone marrow (BMSC), which was also associated with catechol interactions with amine and thiol groups on the cell membrane and with the membrane. Electrical stimulation also affects the cell growth rate on the conductive hydrogel. 279 9.12 | Tissue engineering 9.12.1 | Tissue adhesives Fibrin glue and cyanoacrylates are marketed, and conventional tissue adhesives are often used in surgery. However, poor wet adhesion, poor cytocompatibility, and higher suture costs are disadvantages that lead to new biomimetic tissue adhesives. 230 These adhesives have been inspired by barnacles, caddis fly larvae, sandcastle worms, and mussels. 280 Sealing the tissues, wound closure, and stopping bleeding are some applications of bioadhesives in surgery. Many musselinspired bioadhesives have been designed using MI chemistry. A biomimetic approach has resulted in the design of scaffolds composed of hydroxyapatite and collagen. 287 Adhesion of osteoblast onto the implant's surface determines the implant's biocompatibility. 288 Surface modification was considered a key strategy to increase the aptness of bone implants in orthopedics and dentistry. 289  On the other hand, electrical stimulation can improve the growth rate of axons and neurite differentiation such that conductive materials (e.g., graphene and CNT) have been used for manufacturing scaffolds for nerve regeneration. PCL loaded with 3D porous graphene (SG) or multi-layer graphene (MG) scaffolds were fabricated using a 3D printer and layer-by-layer casting (LBLC) techniques. 316  Newly designed contact lenses, not only can sustain and targeted release of the drugs, more effective curative efficiency at a longer rate than eye drops but also cause smaller side effects.
F I G U R E 2 5 Schematic immobilization of RGD/heparin on dopamine/polytetrafluoroethylene film 309 Due to the unique characteristics of dopamine, such as the functionalization ability, wet adhesion, antimicrobial properties, noninflammatory properties, and high visible light transmittance, MI-compounds are applied widely as a suitable coating for ocular disease treatments.
An in vivo analysis of the multilayer development of silver nanoparticles on the contact lens surface coated by dopamine found that due to the higher release rate of silver cations from the dopamine layer, the antibacterial behavior of the lens and the auto-oxidation of dopamine led to a higher visible light transmission of the lens. 319 Optimizing the silver particle size and concentration, which is covered by the optimal amount of dopamine, exhibits a practical curative impact on bacterial and fungal keratitis hybrid treatments. 319

| CLINICAL APPLICATIONS
Although mussel-inspired biomaterials have vastly been studied and exploited on the lab scale, still several practical challenges are remained unsolved to improve the translation of these compounds for human trials. Noteworthily, one of the most important issues is to protect catechol moieties from unwanted oxidation. Moreover, the biodegradability of PDA is another factor that should be considered to ensure the success of dopamine-conjugated biomaterials for clinical trials. 320,321 Nevertheless, a few numbers of clinical trials on catecholamine chitosan-based hemostatic pads have been undertaken. For instance, InnoSEAL Plus which is introduced by InnoTherapy in 2015 is a coagulation factor-free catecholamine chitosan-based bioadhesive to control intraoperative/postoperative bleeding. The proposed mechanism for this inexpensive product is based on a quick reaction with plasma protein in the blood and simultaneously production of the hemostatic pellicle. This mechanism is independent of the human blood coagulation system and thereby is expected to excel other fibrin-based sealants when it comes to dealing with patients with the disorder in normal blood coagulation. To date, this FDA-approved pad has been subjected to different single and multi-center open-label randomized controlled clinical trials in high-income economy countries such as South Korea, as well as lower-middle-income countries such as Pakistan. 322,323 In 2020, Aijaz et al. 323  Expectantly, ever-increasing in research efforts on musselinspired materials in all fields of biomedical engineering from drug delivery and tissue engineering to cancer research has increased the hope for more patents and FDA-approved products for clinical trials and real-life applications in near future.

| SUMMARY
Understanding MI chemistry as a nature-selected mechanism has opened a new window with limitless applications for broad-ranging of requests from environmental to medicine. From the biomedical application perspective, to develop advanced materials such as wearable devices, drug delivery systems, and surgical glues with strong adhesion to wet surfaces (e.g., chronic wounded tissues) while still keeping high mechanical properties, there is an essential need to acquire profound knowledge about MI chemistry and comprehensively study of distinctive adhesiveness of the mussel foot proteins. In this review article, we summarized and thoroughly discussed different aspects of mussel chemistry and the sophisticated mechanism of wet adhesion which could provide an excellent guideline for the design of future functional materials for diversified biomedical applications.

CONFLICT OF INTEREST
There is no conflict of interest to declare.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.